The 1851 census is
online at the official 'Find my Past' website here.
This is a pay per view site, and is a complete transcription of the census.
You can also search the census for many other criteria, other than just
names and
addresses.

The most accurate way
to access the 1851 census is to visit your local county or regional
archives.

There
is a 'Free Census' online website that eventually aims to have all
of the
censuses
including the 1851, online free to search and view. You can
access it here.

There are numerous part censuses online, and the best way to access these is
via a search on Google (see search box below) using the search phrase "1851
census online", or whichever census year you are searching for.

Lots of family history societies have transcribed this census, which may be
available on cd, and you should refer to the 'Societies' page (accessible
via 'Finding Aids' button) to find a link to their websites.